1. Field of the Invention
The present invention relates to a bolting system for maintaining a clamping force between bolted parts while operating at a temperature of 800xc2x0 F. to 1200xc2x0 F. and more particularly to a fastener construction providing a coefficient of thermal expansion similar to the coefficient of thermal expansion of the bolted parts for minimizing differential dimension changes due to temperature variations and providing a shank portion of the fastener with an elastic limit sufficiently great to maintain a clamping force between the bolted parts throughout a temperature range from ambient to the operating temperature.
2. Description of the Prior Art
The present invention is addressed to solving complex problems involving bolting systems required to operate at high temperatures for an extended operating time. Conventional bolting systems used to form high temperature joints, used particularly in steam turbines and similar joints, have the acute disadvantage that they become considerably weaker when the operating temperature increases. It is necessary in such a bolting system that components are designed to clamp the joint firmly without damaging the most expensive part, the main turbine casting. In some cases the clamped part may be an expensive valve body or any other component. While the present invention is not so limited, a bolting system for a main steam turbine casing has been chosen for the purpose of disclosing the present invention.
At elevated temperatures of the order of 800xc2x0 F. to 1200xc2x0 F., bolting parts, typically studs tensioned in a threaded tapped hole, often fail because of an improper selection of stud material. The studs soften, stretch and become loose. Other problems occur because the stud is subjected to differential expansion causing the clamped part to become loose when the stud expands faster than the clamped part and similarly the stud becomes over stressed when stud expansion is slower than the clamped part. In the latter circumstance conventional studs stretch permanently under the clamping force because the force is too great.
By way of a detailed example of such bolting problems, is the bolting of the split casing of a steam turbine. The casing in the area around the steam inlet reaches a temperature typically of 1000xc2x0 F. to 1050xc2x0 F. The turbine casing is usually heavy with wall thickness ranging from two to eight inches thick formed by casting a chrome molybdenum vanadium steel. This cast steel is relatively strong at 1000xc2x0 F. compared to other low cost casting materials. Relatively strong means it will soften at 1000xc2x0 F. to less than 10,000 psi creep strength while other low cost castings soften to 2,000-5,000 psi creep strength at 1000xc2x0 F. Most casting materials start at about 40,000 psi to 60,000 psi yield strength when cold. Steam turbines have to withstand not only high temperatures, but also high pressures. The high temperatures, high pressures and relatively soft materials demand that the castings are made with very thick walls. The larger the turbine the thicker the wall.
Since a turbine housing is split along its axis, the two halves have to be bolted together. On large turbines the bolts may be as large as 6 inches in diameter. On small turbines the bolts may be 2 inches in diameter. In many cases the bolts are made from a material similar to the material of the housing, which in turn allows both, bolt as well as housing, to expand at the same rate as the temperature rises. This has been the practice for many years when the chrome molybdenum vanadium steel casting was bolted together with chrome molybdenum vanadium steel bolts. By design the combined cross section of the chrome molybdenum vanadium steel bolts is smaller than the cross section of the cast housing. After a certain time the bolts relax to a point where they do not clamp the two castings sufficiently causing the turbine to leak steam. Turbine designers try to estimate the time it takes for the bolts to relax sufficiently to cause leaks. A five year period was usually considered the time span before leaks occurred and corresponds to scheduled turbine overhaul. However, the estimated time period was not always accurate. Presently, operators prefer to extend the time between overhauls and also raise the operating temperature by up to 50xc2x0 F. to improve efficiency.
To solve the above problems turbine designers and operators attempted to improve performance by using substitute bolting materials. Some improved performance was obtained by the addition of trace elements such as boron to the basic chrome molybdenum vanadium steel, but the improvements obtained were not sufficient. Many substitute materials were strong enough to operate at the required high temperatures but the expansion coefficient of the bolting material did not match that of the steel casting material. Many heat resistant stainless steels expand at a rapid rate causing a reduction and in extreme circumstances an actual loss of the clamping force provided by the tightening process when reaching the operating temperature of the turbine. The following example demonstrates the problem encountered due to differential expansion at high temperature between bolting material and the turbine casting:
This example demonstrates that the expansion of the bolt exceeds the expansion of the flange by 0.018 inches. Tensioning the bolt to create a preload will not resolve the problem. Expansion of the bolt, because of the preload, at room temperature produces an elongation according to Hook""s Law:
The bolt was stretched 0.015 inches by the tension causing the preload but the bolt expanded 0.018 inches due to thermal expansion from 70xc2x0 F. to 1000xc2x0 F. Thus the bolt is now loose by 0.003 inches which is obviously not a workable solution.
Another bolting material was found to be unsuitable and that is a martensitic stainless steel of the 400 series. The addition of vanadium to the basic alloy creates an alloy with about twice the strength at high temperatures as compared with the chrome molybdenum vanadium bolting material. The improved 400 series stainless steel has AISI designation of 422 and is known in the industry as xe2x80x9c12% chrome steelxe2x80x9d. This chrome steel used as a bolting material in Example 1, having a thermal expansion coefficient of 6.4xc3x9710xe2x88x926, results in the expansion of the 10 inches bolt by 6.4xc3x9710xe2x88x926xc3x9710xc3x97930=0.059 inches. The expansion of the flanges of the housing is 0.073 inches and therefore the bolt must stretch 0.014 inches in excess of the 0.015 inches resulting from the tightening procedure. The total stretch is now 0.015 inches+0.014 inches=0.029 inches. Using Hook""s law the tension in the bolt would now be:
0.029xc3x9730xc3x97106/10=87,000 psi
The stress increased by a factor of about 2. In reality Hook""s law does not apply when materials are stressed above their elastic limits. The 12% chrome material is much too soft at 1000OF to sustain the 87,000 psi stress. Under these operating conditions the 12% bolting material can sustain only about 18,000 psi of stress over long periods (10,000 hours or more). When over-stressed, the bolt will stretch permanently and therefore loose the tension producing the preload. Another problem arises when the joint of the housing cools down. The casting will shrink faster than the bolt (caused by the different coefficients of expansion) and the bolt will become loose. It will have to be re-tightened before the turbine can be started up again. During the following operation of the turbine the high temperature will stretch the bolt again. Soon the bolt has stretched to such an extent that the threads no longer fit the threads of the nut and a replacement is necessary. Another problem with 12% chrome steel is that the material becomes brittle and cracks during the use. A failure of more than one bolt can lead to catastrophic consequences of the steam turbine.
A nickel super alloy such as Inconel 718 has the desired properties for bolting materials required to operate at temperatures of 1000-1050xc2x0 F. but a completely new approach to the bolted joint design is needed to successfully operate in the high temperature and high pressure environment in the operating steam turbine. Nickel super alloys will not significantly soften at high temperatures and these materials are approximately 10 times stronger at 1000xc2x0 F. than the chrome molybdenum vanadium steel and approximately 5 times stronger than the 12% chrome steel. The super alloy has a coefficient of thermal expansion closely matching the coefficient of thermal expansion of the turbine housing casting.
Because of the strength of the material, the design of the fastener must address a whole new set of problems. One particular problem is the method of tightening the fastener to provide the desired clamping force at both ambient and operating temperatures. Thermal tightening is not reliable and large preloads are difficult to obtain. Studs in turbine casings are spaced very closely and hydraulic tensioners cannot be used due to insufficient space and for the same reason hydraulic wrenches are not suitable. Another serious problem when using super alloys is that the extremely strong bolts can now deform the turbine casting. Assume that a turbine is heated to 1000xc2x0 F. and the bolt was tightened to 45,000 psi. Due to the same expansion coefficient of the bolt and housing the clamping force does not change. The bolt is strong, it has a creep strength of 100,000 psi even at 1000xc2x0 F., but at 1000xc2x0 F. now the housing has a creep strength of less than 10,000 psi. Something has to give, in this case it is the housing. The 100,000 psi bolt can easily deform the 10,000 psi housing causing serious damage.
It is an object of the present invention to provide a bolting system that will maintain a predetermined clamping force while in use at elevated temperatures in the range of 800xc2x0 F. to 1200xc2x0 F. and more particularly at a typical temperature of 1000xc2x0 F. over a long time period without damage to the bolted parts.
It is a further object of the present invention to provide a fastener having a stress releaving threaded joint with the material of the bolted part for transferring stress from the fastener to the relatively low strength material of the bolted part without damage to the threaded connection in operating conditions of extreme thermal excursions.
It is another object of the present invention to provide a fastener system featuring a large load bearing support for a bolted part transverse to the path of stress in the fastener to avoid upsetting the material of the bolted part in operating conditions of extreme thermal excursions.
According to the present invention there is provided a method for maintaining a clamping force between bolted parts while operating at a temperature of 800xc2x0 F. to 1200xc2x0 F., the method including the steps of selecting an elongated fastener having a stress generator and a connector at opposite ends of a shank portion, the shank portion having a coefficient of thermal expansion similar to the coefficient of thermal expansion of such bolted parts to minimizing differential dimensional changes due to temperature variations, the selected shank having an elastic limit sufficiently great for maintaining a clamping force between such bolted parts throughout a temperature range from ambient temperature to an operating temperature of between 800 degrees F to 1200 degrees F, the selected fastener shank having a creep strength several times greater than the creep strength of such bolted parts at the operating temperature; installing the selected fastener to mechanically join such bolted parts, and operating the stress generator substantially at an ambient temperature to stress the shank part sufficiently within elastic limits thereof to maintain a clamping force at the operating temperature of between 800 degrees F to 1200 degrees F.